Duplicate PackageReferences are create for MsTest

[bkqc]

Problem:

I have a project in which I was already referencing MSTest.TestAdapter and MSTest.TestFramework both at version 2.2.10.
After conversion, I now have a new ItemGroup PackageReferences for MSTest.TestAdapter and MSTest.TestFramework version 1.4.0 and a new reference to Microsoft.NET.Test.Sdk 16.0.1.

At first I thought my project always was at 1.4.0 so I tried to update it with the package manager but it always fell back to 1.4.0. That's when I realised there was a duplicate.

Suggestion:

Detect if the package is already there and if it is, don't add it.